● Factor XIII is a transglutaminase that circulates as a zymogen comprised of 2 catalytic A subunits and 2 carrier B subunits
● The A subunit is synthesized in platelets, monocytes and macrophages while the B subunit is synthesized in the liver; the A and B dimers then assemble in the plasma to form a heterotetramer
● Factor XIII is activated by thrombin and is responsible for catalyzing the final step in the coagulation cascade by cross-linking fibrin (in the presence of calcium)
● Deficiency is due to a defect in either the A gene (type 2) or B gene (type 1)

● Inherited as an autosomal recessive trait
● Most cases are due to mutations in A subunit gene on chromosome 6
● More than 70 mutations have been identified, most of which are missense and nonsense mutations
● Only 5 mutations in FXIII B deficient patients have been identified; gene is on chromosome 1

Clinical features
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● Normal PT, PTT, thrombin time, fibrinogen
● Screening test for factor XIII deficiency uses the clot solubility test in which patient plasma is incubated with thrombin and calcium; deficiency will cause the clot to dissolve in the presence of urea or acid
● A standard mixing test using patient plasma and normal pooled plasma is usually performed to rule out the presence of an inhibitor
● Confirmatory testing uses a quantitative factor XIII activity assay

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